As to a flexible metal-clad laminate which is usually used as a material for a flexible printed circuit board, a three-layered flexible metal-clad laminate comprising all-aromatic polyimide film, adhesive and copper foil has been generally known and has been used mostly in general-purpose use. On the other hand, a two-layered flexible metal-clad laminate constituted only from polyimide and copper foil without an adhesive has been also known and that which is called a metallizing type where copper is directly formed by a metal plating to a polyimide film, that which is called a casting type where a polyimide varnish is applied to copper foil and that which is called a laminated type where a thermoplastic polyimide and copper foil are adhered by means of thermal pressure connection have been known. They have been widely used at present as the parts for electronic instruments where flexibility and small occupying space are required being exemplified by a device-installed substrate for a display apparatus such as liquid crystal display and plasma display and by a connecting cable between substrates and operation switch substrate for mobile phones, digital cameras, portable game machines, etc.
Moreover, as a development of bendable paper displays and the like in recent years, necessity for a transparent film substrate in place of the current glass substrate is increasing and application of a flexible printed circuit board as a transparent film substrate is being considered.
In order to apply a flexible printed circuit board to such a use, it is necessary that a flexible printed circuit board and a flexible metal-clad laminate which is a raw material therefor has a colorless transparency like glass in addition to the conventional heat resistance and flexibility. Unfortunately however, the commercially available all-aromatic polyimide (such as APICAL manufactured by Kaneka Corporation) which is currently used for a flexible printed circuit board and also for a flexible metal-clad laminate which is a raw material therefor at present is colored in yellowish brown due to intramolecular and intermolecular formation of a charge-transfer complex and have difficulty in applying to the use where colorless transparency is necessary such as a transparent film substrate.
It has been known that, in order to make polyimide colorless and transparent, an alicyclic diamine or an aliphatic diamine is used as a diamine component to suppress the formation of intramolecular and intermolecular charge-transfer complex. For example, in Japanese Laid-Open Patent No. 2002-161136, there is a proposal for a polyimide which is produced by imidation of a polyimide precursor (polyamidic acid) formed from an aromatic acid dianhydride such as pyromellitic acid dianhydride or 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride and trans-1,4-diaminocyclohexane. Although said polyimide shows high-heat resistance and high transparency, rigidity and linearity of the polyimide main chain skeleton are high and, therefore, there is a problem that elongation is low and flexibility is lacking.
In Japanese Laid-Open Patent No. 010993/95, there is a proposal for a colorless and transparent copolymerized polyimide which is formed from an alicyclic diamine having high flexibility such as 4,4′-methylenebis(cyclohexylamine) and a specific aromatic acid dianhydride such as 3,3′,4,4′-benzophenonetetracarboxylic acid dianhydride or 3,3′,4,4′-diphenylsulfonetetracarboxylic acid dianhydride.
However, glass transition temperature of any of the resulting polyimides is lower than 270° C. and the product can hardly be said to well satisfy the heat resistance. In addition, there is a problem that, in using it as a flexible printed circuit board, its thermal expansion coefficient is high. Furthermore, the proposed polyimide has a poor solubility and it is necessary to be subjected to a heating treatment at high temperature after a molding process (after applying) in a form of a polyamic acid which is a precursor and, therefore, in a continuous production of a flexible metal-clad laminate using said resin, there is a problem that the productivity becomes low, an expensive equipment is necessary and the manufacturing cost becomes high.